1. Field of the Invention
The present invention relates to a trellis-coded modulation (TCM) decoder and a decoding method for use in the TCM decoder. More particularly, the present invention relates to a TCM decoder for use in a high-definition television (HDTV) receiver and a decoding method therefor.
2. Description of the Related Art
In general, for a large-screen, high resolution TV, a grand alliance-high definition television (GA-HDTV)has been developed in the United States, and a vestigial side band (VSB) modulation method for digital transmission has been adopted as a modulation method for the GA-HDTV. Such a GA-HDTV adopting the VSB modulation method is called a "GA-VSB" system. An 8-VSB modulation method using 8 levels has been used for the GA-HDTV for a terrestrial broadcast mode. A 16-VSB modulation method using 16 levels for a high-speed cable mode has been used for the GA-HDTV.
One of characteristics of the GA-VSB system, as the standard for an HDTV for the US-type terrestrial broadcasting, is a TCM method used in order to increase noise immunity. The TCM refers to a modulation method having an error correction function based on a conventional modulation method, by which performance in transmission can increase without increasing channel bandwidth.
The structure of a TCM encoder of a GA-VSB system is shown in FIG. 1A. In FIG. 1A, the TCM encoder receives two bits I.sub.1 and I.sub.2 as inputs. A convolution encoder 106 receives one bit I.sub.2 and outputs two bits O.sub.2 and O.sub.3, and a precoder 100 receives the remaining bit I.sub.1 and outputs one bit O.sub.1. The precoder 100 is another feature of the GA-VSB system and is used to cope with the use of an National Television System Committee (NTSC) rejection filter by a receiver. Thus, when a total of three bits O.sub.1, O.sub.2, and O.sub.3 are input to a mapper 114. As shown in FIG. 1B, the mapper 114 produces one output symbol value M.sub.OUT, at one of 8 levels, which has a relationship of one-to-one correspondence with the combination of inputs O.sub.1, O.sub.2, and O.sub.3. Here, delay units 104,108 and 112, designated by "D", delay input data by a 12-symbol period, which provides the same effect as when twelve encoders each having a unit delay, arranged in parallel, are used. This is called "12-symbol interleaving". The adverse affects of burst-type noise can be reduced, and the state number of the TCM decoder, which increases when the NTSC rejection filter is used at a receiver, can be decreased by means of the 12-symbol interleaving.
Assuming that the TCM encoder shown in FIG. 1A is a single encoder having a unit delay, the operation of the TCM encoder when the previous states of the delay units 108 and 112 are "00" will be described.
When the least significant bit (LSB; I.sub.2) of the two bit parallel input is "0", the next state of the convolution encoder 106 which has received the LSB (I.sub.2) of "0" becomes "00", and the two bit output O.sub.2 O.sub.3 of the convolution encoder 106 becomes "00". The output of the mapper 114 is determined according to the most significant bit (MSB; I.sub.1) of the two bits input in parallel. When the MSB (I.sub.1) is "0", that is, the input I.sub.1 I.sub.2 of the TCM encoder is "00", the output of the mapper 114 becomes "-7(000)". Also, when the MSB (I.sub.1) is "1", that is, the input I.sub.1 I.sub.2 of the encoder is "10", the output of the mapper 114 becomes "1(100)".
On the other hand, when the LSB (I.sub.2) of the two bits input in parallel to the encoder is "1", the next state of the convolution encoder 106 which has received the LSB (I.sub.1) of "1" becomes "01" and the output of the convolution encoder 106 becomes "10". The output of the mapper 114 becomes "-3(010)" or "5(110)" according to the logic state of the MSB (I.sub.1) input to the precoder 100.
Such states of the TCM encoder shown in FIG. 1A are shown as a trellis diagram in FIG. 2. The number of memories (corresponding to delay units) of the convolution encoder 106 of FIG. 1A equals two, thus the total number of states is equal to four. Since one bit remains without going through the convolution coding, the number of parallel paths, representing the number of possible transitions into another state, becomes two. For example, if the previous state is "10", the transition into the next state "00" occurs when the input data I.sub.1 I.sub.2 of the encoder is "01" or "11", resulting in two parallel paths.
FIG. 3 is a diagram showing a data frame format of the GA-VSB system. One frame of the VSB data is comprised of two fields, and each field is comprised of a field synchronous segment (hereinafter referred to as a "sync segment") and 312 data segments. Each data segment is comprised of 4 segment sync symbols and 828 data symbols. Four segment sync symbols are also inserted into the 8-level digital data stream at the beginning of each of the field sync segments as well as each data segment. The segment sync symbols are to be used for timing restoration. Here, the segment sync is formed in a predetermined pattern where 4 symbols have the signal levels +5, -5, -5 and +5, respectively, and the remaining data has arbitrary signal levels among the 8 levels .+-.1, .+-.3, .+-.5 and .+-.7. In addition to the segment sync symbols, each field sync segment, corresponding to the first segment of each field, includes a field sync signal (FIELD SYNC #1 or FIELD SYNC #2) which indicates the start of the field. The field sync signal sequence is used for equalization and error correction decoding, thus the TCM encoder does not perform coding during that period.
The overall structure of the TCM encoder of the actual GA-VSB system, in consideration of a general segment/field sync, is shown in FIG. 4. In FIG. 4, reference numeral 120 represents a precoder, reference numeral 128 represents a convolution encoder, reference numeral 140 represents a mapper, and reference numeral 142 represents a sync inserter. Respective delay units 124, 132 and 138, each constituted of a shift register, receive their own output during the segment sync period via multiplexers (MUXs) 126, 130 and 136, respectively, according to a segment sync timing signal. Thus, data before and after the 12 symbols from the segment sync is encoded while being connected together. This is the same as the case where each encoder receiving the segment sync, in 12 TCM encoders arranged in parallel, holds the data during the segment sync and performs encoding after the next data is input. Additionally, a multiplexer (MUX) of the sync inserter 142 selects 4-symbol segment sync with a predetermined pattern of +5, -5, -5 and +5 during the segment sync period, and selects the TCM coded data, output from the mapper 140, during the other period, according to the segment sync timing signal.
On the other hand, the operation of the TCM encoder on the field sync is different from that when the segment sync is input, since the field sync period comes to 828 symbols while the segment sync period comes to 4 symbols. Thus, delay units 126, 132 and 138 of the TCM encoder continuously hold the input data during the whole field sync segment (including segment sync) until the data of data segment is input.
As above, the structure of the TCM encoder of the GA-VSB system, shown in FIGS. 1A through 4, is disclosed in the following reference [1]: Grand Alliance HDTV system Specification, submitted to the ACATS Technical Subgroups in February 1994. However, the structure of a TCM decoder is not disclosed yet.